Power/Performance Bits: Dec. 30

Harvard researchers create vivid optical effects on paper that may find use in optoelectronic devices like photodetectors and solar cells; EPFL scientists have developed a technique to uncover the way cuprates become superconductors, which could translate to low-cost electricity without energy loss.

Crafting ultrathin color coatings
Harvard University researchers have developed a technique that coats a metallic object with an extremely thin layer of semiconductor, just a few nanometers thick. And while the semiconductor is a steely gray color, the object ends up shining in vibrant hues because the coating exploits interference effects in the thin films. Carefully tuned in the laboratory, the coatings can produce a bright, solid pink—or, say, a vivid blue—using the same two metals, applied with only a few atoms’ difference in thickness.

Interestingly, the technique also works on materials such as a piece of notebook paper, the researchers said, which suggests that the ultrathin coatings could be applied to essentially any rough or flexible material, from wearable fabrics to stretchable electronics.

Due to the metal coatings absorbing a lot of light, reflecting only a narrow set of wavelengths, the researchers believe they could also be incorporated into optoelectronic devices like photodetectors and solar cells.

The fact that these can be deposited on flexible substrates has implications for flexible and maybe even stretchable optoelectronics that could be part of clothing or could be rolled up or folded, they added.

Electron spin could be the key to high-temperature superconductivity
EPFL scientists have used a cutting-edge technique to uncover the way cuprates — materials with great promise for achieving superconductivity at higher temperatures — become superconductors. This could translate to low-cost electricity without energy loss, with the eventual goal of developing room-temperature superconductors.

The researchers explained that conventional superconductors are materials that conduct electricity with no electrical resistance under temperatures nearing absolute zero (−273.15°C or 0 Kelvin), and that under these conditions, the electrons of the material join up and form electron couples that are called “Cooper pairs”, and in this form can flow without resistance. Generally, Cooper pairs form at such low temperatures, and only when the superconductor’s atoms vibrate and create an attractive force between electrons.
But there is a copper-based material in a class of superconductors where Cooper pairs do not form because atoms nudge them together called “cuprates,” which, in normal temperatures, are actually electrical insulators and magnets.

The popularity of cuprates comes from the fact that they become superconductors at much higher temperatures than other materials: just over -123.15°C (150 Kelvin) making them a good path towards everyday superconductivity.

Since previous studies have suggested that cuprates do not become superconducting like other materials, a team of researchers at EPFL set out to discover how superconductivity arises in cuprates.

They found that when they become superconducting, cuprates do not lose their magnetic properties and that something of the magnet remains in the superconductor, and could play a major role in the appearance of superconductivity.

The researchers said that these findings propose a novel understanding of superconductivity in cuprates, and possibly in other high-temperature superconductors, and that by revealing the role of spin interactions, it might pave the way for bringing high-temperature superconductors into the real world.